[PATCH v4 08/14] locks: avoid taking global lock if possible when waking up blocked waiters

From: Jeff Layton
Date: Fri Jun 21 2013 - 09:03:59 EST

Since we always hold the i_lock when inserting a new waiter onto the
fl_block list, we can avoid taking the global lock at all if we find
that it's empty when we go to wake up blocked waiters.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
fs/locks.c | 15 ++++++++++++++-
1 files changed, 14 insertions(+), 1 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index ce302d4..84e269f 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -548,7 +548,10 @@ static void locks_delete_block(struct file_lock *waiter)
* the order they blocked. The documentation doesn't require this but
* it seems like the reasonable thing to do.
- * Must be called with file_lock_lock held!
+ * Must be called with both the i_lock and file_lock_lock held. The fl_block
+ * list itself is protected by the file_lock_list, but by ensuring that the
+ * i_lock is also held on insertions we can avoid taking the file_lock_lock
+ * in some cases when we see that the fl_block list is empty.
static void __locks_insert_block(struct file_lock *blocker,
struct file_lock *waiter)
@@ -576,6 +579,16 @@ static void locks_insert_block(struct file_lock *blocker,
static void locks_wake_up_blocks(struct file_lock *blocker)
+ /*
+ * Avoid taking global lock if list is empty. This is safe since new
+ * blocked requests are only added to the list under the i_lock, and
+ * the i_lock is always held here. Note that removal from the fl_block
+ * list does not require the i_lock, so we must recheck list_empty()
+ * after acquiring the file_lock_lock.
+ */
+ if (list_empty(&blocker->fl_block))
+ return;
while (!list_empty(&blocker->fl_block)) {
struct file_lock *waiter;

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